2. Examples

Table 1. Differences

VB6

Tcl/Tk 8.3

Notes/differences

dim a as integer
dim b as integer
a=1 : b=0

set a 1; set b 0

Separator for multiple commands per line. Tcl uses a semicolon.
Multiple commands per line is generally considered bad form, but
the semicolon is also used to implement partial-line comments, so
it is illustrated here.

Partial-line comment. Note the semicolon, used as if the comment is
another command on that line.

dim s as string
s="/data/docs/vb6_to_tcl.htm"

set s {/data/docs/vb6_to_tcl.htm}

Assignment of a string quoted with braces. Most Tcl substitutions
are NOT done in a string quoted with braces. If the string contains
variables or other items that would be substituted, these will be
deferred, but may be substituted at a later time. This is often
done by commands that implement control structures, such as
'if' or 'while'. Once you start to get familiar
with Tcl initially, try to thoroughly understand this process
because it's important to getting 'good' in Tcl.

(No equivalent)

set s "/data/docs/vb6_to_tcl.htm"

Assignment of a quoted string. All Tcl substitutions (variables,
commands, backslashes) are available within a quoted string.

(No equivalent)

set s /data/docs/vb6_to_tcl.htm

Assignment of an unquoted string. All Tcl substitutions (variables,
commands, backslashes) are available within an unquoted string. The
interpreter simply takes the string as the third word in the set
command (second argument to the set command). This works if there
is no whitespace or certain other characters in the string. Use
judiciously, especially when dealing with arbitrary data entered by
the user.

set s {
Free software is not just
about being 'free of charge'
but about freedom to create
and use the best possible tools.
}

Assignment of multi-line string. Note the more cluttered syntax in
VB, which makes it more difficult to read than the Tcl code.

dim s as string
dim t as string
s = trim(t)

set s [string trim $t]

Assignment of function return value. The third word of this set
command is surrounded in square brackets. That means it is itself a
command to be executed, with the result taking its place as the
third word of the set command.

dim s as string
dim t as string
s = lcase(trim(t))

set s [string tolower [string trim $t]]

Assignment of function-of-function.

dim x as double
dim y as double
x = (y + 10) * 5

set x [expr {($y + 10) * 5}]

Assignment of result of a mathematical expression. The Tcl
interpreter relies on the expr command to evaluate mathematical or
logical expressions. Many other commands such as 'if' or
'while' also rely on expr in their implementation. When
used explicitly, expr should be passed a single argument which is a
string containing the expression (as shown here). That could get
cumbersome in simple cases where you just want to add a certain
increment to a variable. Try using the incr command for that
instead.

dim s as string
s = s &"more text"

append s {more text}

Append to an existing string. This is one of the slowest operations
in VB, but is typically very speedy in Tcl. Speed is important here
because it is often done within loops or compound loops.

Procedure definition. Note that VB uses a separate syntax for subs
and for functions. Tcl uses the proc command to define
either one. proc itself is an ordinary Tcl command that
executes like any other command. Its first argument is a Tcl list
of the parameters of the new procedure. Its second argument is a
large string containing the body of the new procedure (actual Tcl
script). Important: Tcl is case sensitive in
almost all operations, including all references to command names
and variable names, as well as (by default) string data
comparisons. So a call to Proc would cause an error
(capital P), as would a call to My_Sub, or a reference to
the variable B within my_sub (b was
defined as lower case).

dim i as integer
if i < 0 then i = 0 else i = i - 1

if {$i < 0} {set i 0} {incr i -1}
# alternate form
if {$i < 0} then {set i 0} else {incr i -1}
# another alternate form
if {$i < 0} then {
set i 0
} else {
incr i -1
}

'if' conditional execution. The Tcl 'if' command
ignores the optional keywords 'then' and 'else' if
they are present. Since both code blocks are just strings, they can
be enclosed in braces and nicely formatted as shown. To avoid
syntax errors, also enclose any non-trivial test expression in
braces. That way substitutions (such as $i here) are deferred until
the 'if' command passes the test expression to the
expression parser.

dim i as integer
i = 1
while i < 2000
i = i * 2
wend
'alternate form
i = 1
do while i < 2000
i = i * 2
loop

set i 1
while {$i < 2000} {
set i [expr {$i * 2}]
}

'while' loop. This is similar to the Tcl 'if'
command in that it takes a test expression as its first argument,
followed by a string of code.

'for' loop with an integer counter. In Tcl (or any other
language) this is equivalent to a 'while' loop. In some
languages such as VB, 'for' is not as flexible as
'while'. In Tcl this is not the case. Anything can be used
as the initialization code, the test-for-continuation expression,
and the increment code. Those pieces are not restricted to doing
anything in particular, as you can see by the final example.

dim c as new collection
dim o as object
c.add "Mark"
c.add "Roy"
c.add "Brian"
for each o in c
debug.print o
next

set c [list Mark Roy Brian]
foreach o $c {
puts $o
}

Loop through items in a data structure. In Tcl, a list data
structure is used. VB has no direct equivalent to that, but a
collection object is the most similar. Note that VB collections are
far slower than Tcl lists in typical operations due to the overhead
of using method calls to objects. Also note that there are
far more powerful and creative uses of the
foreach command that are not shown here. Those have no direct
equivalent in VB.

One-of-many execution. Note the Tcl version is case sensitive. In
VB it often is not, depending on the 'option compare' that
is in effect for the module. The -exact option specifies
an exact string match is required, as opposed to a pattern match or
regular expression match (this has no bearing on case sensitivity).
Also note that there are more powerful and creative uses of the
switch command that are not shown here.

if [catch {
puts $a ;# a has not been set
} my_err] {
puts "error message: $my_err"
puts "stack trace: $errorInfo"
# these things would have been shown
# by the default error handler anyway.
} else {
puts {All is well.}
# the else block is optional.
}

Error handling. In VB, handling errors concisely can be a problem,
especially if different actions need to be taken based on which
part of the code failed. Tcl catch command neatly solves
these problems. In addition, Tcl automatically provides a stack
trace of the code that failed. In VB, the stack trace has to be
explicitly built by the code, if a stack trace is desired while the
application is in production (not in the IDE). This is an advantage
for Tcl when debugging in the field. Note that catch
returns a boolean 1 or 0, which is typically used with
'if', as shown here.

(No equivalent)

set i [expr $e]

Pass an arbitrary mathematical expression to the interpreter for
evaluation. This could be an expression entered by the user, or
composed by earlier code. This is one of the most powerful aspects
of Tcl. It is not available at all in VB.

(No equivalent)

set s [eval $c]

Pass arbitrary code to the interpreter for execution. This could be
some script entered by the user, or composed by earlier code. This
is one of the most powerful aspects of Tcl. It is not available at
all in VB.

(No equivalent)

source my_script.tcl

Pass an arbitrary filename to the interpreter for execution of that
file as a script. This is one of the most powerful aspects of Tcl.
It is not available at all in VB.

(No equivalent)

set var_name marks_age
incr $var_name

Perform operations on an arbitrarily-chosen variable. The code
shown here will increment the variable marks_age.
Its name (the string "marks_age") is stored in the variable
var_name. In fact, all parts of every command are subject
to one pass of substitution by the interpreter just prior to execution. So any
part of any command (even the name of the command itself) can be
varied based on data or any other criteria. This is one of the most
powerful aspects of Tcl. It is not available at all in VB.

Read whole file into a variable. This VB code is very slow for even
moderately large files. And it has no way to deal with newline
characters in the data. The Tcl code accepts and preserves newlines
in the data. It also normalizes different newline characters into a
single kind of standardized newline character (by default). This
code applies equally well to raw data, or Tcl lists, or Tcl arrays.
The r in the open command indicates
'read' mode.

array set a [list 1 Mark 2 Brian 3 Roy]
set a(4) John
# now some different kinds of
# element names in the same array
set a(Red) Hat
set a(Linux,RedHat) 7.1

Array vs. Array. VB arrays are restricted to using numbers as
subscripts (subscripts, or indexes, are called 'element
names' in Tcl). And the array must be declared to be a certain
size - expanding it requires a (slow) 'ReDim Preserve'
operation. Tcl arrays automatically expand, and they use a
super-efficient hash table implementation to handle even hundreds
of thousands of elements with superior speed. Tcl uses any kind of
data for an element name, and different styles can even be mixed
within the same array. There are no restrictions on the number of
dimensions in each element. Tcl provides simple ways to iterate
through the array, or through only certain elements in the array
(by filter). You can also obtain a full or partial list of the
element names, and do other operations more conveniently than in
VB. To get just a portion of those capabilities in VB requires the
use of a collection or dictionary object. Each of those comes with
its own quirks and pitfalls, such as even higher overhead than a VB
array.

(No equivalent)

array set my_array $my_list
set my_list [array get my_array]

List to array, and back. Easy and rapid translation between these
two primary data structures means that the tools for each one can
be applied to both. They multiply each other's usefulness.

dim a(1 to 100) as string
dim i as integer
dim f_num as integer
f_num = freefile
open "my_file.txt" for output as #f_num
for i=1 to 100
print #f_num, a(i)
next
close #f_num

set f [open my_file.txt w]
puts $f [array get a]
close $f

Write whole array. In this VB code, and frequently in other VB
code, newlines and possibly other characters appearing in the data
will cause errors during a later step (the read-back). This becomes
a problem whenever your code deals with arbitrary data entered by
the user. In Tcl they do not - the data is kept "clean"
at all times. In addition, various combinations of carriage return
(0x0D or decimal 13) and line-feed (0x0A or decimal 10) characters
are automatically normalized by default. Note that these two examples
don't produce identical output files. The Tcl example, like the
VB, writes a plain text file. But the Tcl file will be read back in
(by Tcl) and automatically have the same number of elements, same
element names, etc.. The Tcl list data structure is used for this.
Using it ensures that the data is formatted in a concise,
non-ambiguous, textual representation. It is also readable and
writable by humans.

(No equivalent)

set f [open my_file.txt w]
puts $f [array get a red*]
close $f

Write certain elements of an array. In the VB, a collection or
dictionary object would have to be used for this. A loop would
iterate through all the elements and select them as appropriate. In
the Tcl, the array's name is a and a string pattern
of red* (case sensitive) is used as a filter to select
elements at high speed.

(No equivalent)

set my_list [lsort $my_list]

Sort a list. The sort can be reversed, or ordered by numeric value,
etc. It can also order a list of sublists using an index element.
Tcl contains a full suite of commands for manipulating the list
data structure. See also lappend,
linsert, lreplace,
lsearch, concat, split, join, etc. Tcl lists can also be nested arbitrarily,
and the foreach command has no trouble dealing with that.

package require tclodbc
# assume we have a connection called conn
conn read a "select id, name, age from people"
# processing code goes here
unset a ;# get rid of this array

Retrieve a simple array of data from a database table. In VB data
is always retrieved in a recordset object. In Tcl it can be read
into an array and/or a list, depending on your needs, and the
database package in use.

Complex string pattern search and extraction. Tcl uses
regular expressions for this.
Regular expression is a specification for a
string pattern to be matched, similar in concept to the
wildcard patterns used with VB's 'like' operator,
except on steroids - a whole lot of steroids.
Regular expressions are several times more powerful and flexible
than 'like' patterns. For an informal introduction to
regular expressions, see http://zez.org/article/articleprint/11.
Tcl's regular expression parser is written in hand-optimized
C code and is available to Tcl in several different commands
(regexp, regsub,
lsearch, etc). The simpler, less powerful versions
you're used to are also available for use in several different
commands (glob, string match,
lsearch, and so on). This example
would take 15 to 50 lines of VB code, depending on how robust and
how tolerant of different situations it needs to be. In addition,
that is some of the most difficult, error-prone, and slowest code
that can be written in VB (voice of experience). Here, the code
quickly obtains a list of the URLs of every image on an HTML page.

Complex string pattern search and substitution. Again, Tcl uses
regular expressions. This example would take 40 lines of VB code or
more, especially if it is logically organized with sufficient
comments for a maintenance programmer to follow it. And again, it
is some of the most difficult, error-prone, and slowest code that
can be written in VB. Here the set of three cells in
every row in the HTML body is altered
systematically, while the contents of each cell is preserved.

Implement a network server to answer the client shown above. This
is the complete script. If you're using Wish (the Tcl windowing
shell) this will run all day as shown. If you're using Tclsh
(the console Tcl shell) add a vwait command at the end,
to make the program wait for events instead of terminating at the
end of the script. That difference between the two shells is
necessary and intentional, since Wish is event-driven by default,
and Tclsh is not.